I think it is, or was, a southern thing. I was born and raised in FL before it was transformed into a giant retirement home for yankee retirees. Many of the adults I knew as a kid cooled their coffee in the saucer. Some, like my grand-dad, blew on it to cool it and then poured it back into the cup to let it mix with the hot coffee that was still there. Others simply blew on it a few times and then sipped from the edge of the saucer.
That was in the days long before automatic coffee makers so the coffee was perked on the stove and was boiling hot. With automatic coffee makers today the coffee is not too hot to drink as soon as it's poured, so the old custom seems to have died out even among the older folks down here.
My first wife always put an ice cube in her hot coffee to cool it....
I bought my Mom a one cup, auto turn off pot after I found out she burnt the coffee pot on the gas stove. (She forgot about it.) The coffee temp is perfect, far from "burning hot". I recommend it for every Senior Citizen. (If you can find them)